To keep the input signal of a power amplifier at a desired level and to compensate the distortions with high accuracy by using a gain control amplifier for absorbing gain fluctuations of an amplifier, a filter, a modulator, a mixer, etc., which are due to the changes in operating environment such as temperature, frequency and power voltage.

The input signal of a power amplifier 121 of a radio part 102 is detected by a directional coupler 120, measured at a power measurement part 123 of a control part 103 and compared with the desired power value by a power comparator 124. Based on the result of the comparator 124, a gain control part 125 controls the gain of a gain control amplifier 116 of the part 102, in order to set the input power of the amplifier 121 at a desired value. Thus, the amplifier 116 absorbs the gain fluctuations of elements of quadrature modulator 115, the BPF 117 and 119 and a mixer 118 which are due to the changes in operating environment such as the temperature, frequency and power voltage. As a result, the input signal of the amplifier 121 is kept at a desired level, and the distortions can be compensated with high accuracy.